The present invention relates to a mobile terminal field, and more particularly to a mobile terminal, a base station and a non-transitory computer-readable medium to securely receive signal.
Since low frequency signals are not suitable for long-distance propagation in the air, there is a concept of modulation and demodulation in the mobile communication process. With an analog of the mobile communication to the traditional postal mail, the modulation is the process in which the sender (transmitting party) puts a message (information, a sequence of 0 and 1) in an envelope (carrier wave), and then sends it out (transmitted by the antenna), and the demodulation is a process in which the receiver (receiving party) receives and opens the envelope (removing the carrier wave) to obtain the content of the message (information sent by the transmitting party). In the above discussion, it is assumed that both sides of the communication are determined on a given carrier wave frequency within a given area, that is, the carrier wave frequency is exclusive to both communicating parties. However, the frequency resources are limited. Therefore, when the number of users in a given area is too large, it will not be able to provide more carrier wave frequencies for the newly added communication parties. To alleviate this problem, spread spectrum communication is generated.
Spread spectrum is similar to re-branding an address for the foregoing modulated signal, and all communication parties use the same carrier wave frequency, and the address used for each communication is different from others, so the communication is different from the above by frequency. Therefore, unlike the aforementioned in which the communication is distinguished by frequency, the spread spectrum communication distinguishes the communication by the address, thereby solving the problem of limited frequency resources.
The basic principle of spread spectrum communication is as follows: the base station and the mobile terminal have a series of digital sequence (hereinafter referred to as a PN sequence, i.e., a Pseudo Noise sequence) consisting of 0 and 1 outputted by high frequency cyclic high-speed (the output rate is much higher than the rate of information to be transmitted, such as the voice digital bit rate of 64 kbps, i.e. 64000 bits per second, and the PN sequence output rate of 400 kbps), and when the mobile terminal is in the voice service, the base station allocates a PN sequence start bit to the mobile terminal, and from then on, the data transmission process established by the mobile terminal and the base station is as follows:
It is illustrated that the mobile terminal transmits signals to the base station, the mobile terminal first spreads the voice digit sequence to be transmitted, that is, XORing the voice digit sequence with a PN sequence starting with a start bit allocated by the base station (re-branding an address), and then modulating the same into a carrier wave (enclosing the same into an envelope) and transmitting it by the antenna; when the base station receives the signal, it demodulates (opens the envelope) first, and then only a given PN sequence can be XORed with the demodulated signal, so that the XORed signal becomes a low frequency signal; with the number of the start bit of the given PN sequence in the entire PN sequence (the address re-branded by the transmitting party), it can be known which mobile terminal the signal is transmitted by; certainly, the base station serves multiple mobile terminals at the same time. Then, the PN sequences (different start codes) of respective mobile terminals are respectively XORed with the received signals until a given PN sequence is found to become a low frequency signal after being XORed.
Similarly, for the base station to transmit a signal to a specific mobile terminal, the signal to be transmitted is XORed with the given PN sequence before transmitting out. The mobile terminal receives the signal, and then XORs the PN sequence. If a low frequency signal can be obtained after XORing, it means the signal is transmitted therefor. Otherwise, the signal is not transmitted therefor and then is discarded.
“XOR” is a mathematical operator applied to logical operations. The algorithm is a XOR b=ab+a′b′ (a′ is not a, b′ is not b). The result of true “XOR” false is false, and the result of false “XOR” true is also false. The result of true “XOR” true is true, and the result of false “XOR” false is true. Namely, the two values are the same, the XOR result is true, and on the contrary, is false. The XOR symbol is 0, and the XOR truth table is as follows:
aba b001111010100
A simple illustration will be given to illustrate the process of spreading and despreading in the prior art;
Spreading: as shown in FIG. 1. FIG. 1 is a schematic diagram of a process of obtaining a spread sequence after XORing a low frequency digital signal and a PN sequence in the prior art. Assuming that the low frequency digital signal (information) is 110, a spread sequence is obtained by being XORed with the PN sequence, and then the spread sequence is transmitted; In FIG. 1, the code outputting rate of the PN sequence is six times faster than that of the low frequency digital signal.
Despreading: as shown in FIG. 2. FIG. 2 is a schematic diagram of a process of obtaining a low frequency original signal after a signal receiver receives a spread signal to be XORed with a given PN sequence in the prior art. When the receiving party receives the spread signal to be XORed with the PN sequence to obtain the low frequency original signal (information) 110; when receiving the signals from others, it is still a high frequency signal after being XORed with the given PN sequence. As shown in FIG. 3. FIG. 3 is a schematic diagram of a process that after being XORed with the given PN sequence, it is still a high frequency signal when a signal receiver receives a signal from others in the prior art. Even if there is only one bit different (marked in red, i.e., the first bit), it is still a high frequency signal after being XORed with the PN sequence. Namely, any desired signal will be despreaded to be a low frequency signal, and any unwanted signal is despreaded to be a high frequency signal, which eventually is discarded.
In the communication process, one base station may communicate with multiple mobile terminals at the same time, and the base station uses different PN sequences to communicate with different mobile terminals. However, in fact, each mobile terminal uses the same PN sequence (it just constantly repeats itself), and each mobile terminal uses one PN sequence but starts with different start bits, accurately speaking, it starts with the bits of different serial numbers in the PN sequence. For instance, the PN sequence is 101100110101110011, which is cyclically outputted from the first bit to be a PN sequence that communicates with the base station and the first mobile terminal, and is cyclically outputted from the second bit to be a PN sequence that communicates with the base station and the second mobile terminal, and is cyclically outputted from the third bit to be a PN sequence that communicates with the base station and the third mobile terminal. Therefore, in spread spectrum communication, there is only one long, continuously cycling PN sequence, which is used by all base stations and mobile terminals, and the only difference is that the start bits are in different positions in the PN sequence, and each time a communication is established, the base station allocates a start bit for the mobile terminal and then informs the mobile terminal.
The prior art has the following drawbacks: when the communication is established, the PN sequence is determined, and the lawbreakers can test the received signal by using different start bits for the received signal. The start bit of the PN sequence used in the communication spread spectrum can be obtained to monitor the communication as long as the communication time is long enough.
Therefore, the prior art has yet to be improved and developed.